There is a thread that explains that and some other stuff as well about how Sponge will perform with Non Modded-Modded clients situations and some other stuff:
Sponge, a coremod?And has vanilla Forge…With Plugins?
So since Sponge will be a coremod you will setup a Forge server, but that Forge server will allow non-modded clients to join.
And there are 2 types of Sponge servers then:
-With Mods(like Thermal Expansion,Tinkers Construct, FoodPlus…) and they require the Player that wants to join the server to install them on his client.
-With Plugins(like Essentials,Spleef,GroupManager…) and they will allow you to log onto the server like any other vanilla server and use those plugins properly!Just open you normal Minecraft with the version of that server and join it!Just like a Bukkit server.
However, if the server requires you to install mods, for instance A and B, you will have to install A and B to your Client to join the server!